Gas Laser Apparatus, Laser Gas Temperature Control Method, and Electronic Device Manufacturing Method

The present application is a continuation application of International Application No. PCT/JP2023/007847, filed on Mar. 2, 2023, the entire contents of which are hereby incorporated by reference.

The present disclosure relates to a gas laser apparatus, a laser gas temperature control method, and an electronic device manufacturing method.

In recent years, an improvement in resolutions of semiconductor exposure apparatuses has been desired with miniaturization and higher integration of semiconductor integrated circuits. For this purpose, exposure light sources that release light having a shorter wavelength have been developed. For example, a KrF excimer laser apparatus that outputs laser light having a wavelength of about 248 nm and an ArF excimer laser apparatus that outputs laser light having a wavelength of about 193 nm are used as gas laser apparatuses for exposure.

Spectral linewidths of spontaneous oscillation light of the KrF excimer laser apparatus and the ArF excimer laser apparatus are as wide as 350 pm to 400 pm. Therefore, chromatic aberration may occur if a projection lens is formed of a material that transmits ultraviolet light such as KrF and ArF laser light. As a result, resolving power may be degraded. Thus, a spectral linewidth of laser light output from the gas laser apparatus needs to be narrowed to the extent that chromatic aberration can be ignored. For this purpose, a line narrowing module (LNM) including a line narrowing element (etalon, grating, and the like) may be included in a laser resonator of the gas laser apparatus to narrow the spectral linewidth. Hereinafter, a gas laser apparatus with a narrowed spectral linewidth will be referred to as a line narrowing gas laser apparatus.

A gas laser apparatus according to an aspect of the present disclosure is a gas laser apparatus that outputs pulsed laser light and includes a laser chamber that accommodates laser gas, a discharge electrode that is disposed inside the laser chamber and is configured to cause discharge-excitation of laser gas, an optical element that is disposed on an optical path of the pulsed laser light, and a processor configured to change a target temperature of the laser gas based on either a number of pulses of the pulsed laser light or an elapsed time during which the pulsed laser light is output.

A laser gas temperature control method according to another aspect of the present disclosure is a laser gas temperature control method for a gas laser apparatus that outputs pulsed laser light, the gas laser apparatus including a laser chamber that accommodates laser gas, a discharge electrode that is disposed inside the laser chamber and is configured to cause discharge-excitation of laser gas, an optical element that is disposed on an optical path of the pulsed laser light, and a processor, the method including, by the processor, changing a target temperature of the laser gas based on either a number of pulses of the pulsed laser light or an elapsed time during which the pulsed laser light is output.

An electronic device manufacturing method according to another aspect of the present disclosure includes generating laser light with a gas laser apparatus including a laser chamber that accommodates laser gas, a discharge electrode that is disposed inside the laser chamber and is configured to cause discharge-excitation of laser gas, an optical element that is disposed on an optical path of the pulsed laser light, and a processor configured to change a target temperature of the laser gas based on either a number of pulses of the pulsed laser light output by the discharge-excitation or an elapsed time during which the pulsed laser light is output, outputting the laser light to an exposure apparatus, and exposing a photosensitive substrate to the laser light within the exposure apparatus to manufacture an electronic device.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The embodiments described below show some examples of the present disclosure and do not limit contents of the present disclosure. Not all configurations and operations described in each embodiment are necessarily essential as configurations and operations of the present disclosure. Note that the same components will be denoted by the same reference signs and repeated description thereof will be omitted.

illustrates a configuration diagram of a gas laser apparatusaccording to a comparative example. The comparative example of the present disclosure is an example that the applicant recognizes as known only by the applicant, but is not a publicly known example that is recognized by the applicant. The gas laser apparatusis an excimer laser apparatus that includes a laser oscillator system, a laser gas supply and exhaust system, and a laser control processor.

The laser oscillator systemincludes a laser chamber, a line narrowing module (LNM), an output coupling mirror (output coupler: OC), a power monitor, and a charger.

The laser chamberaccommodates laser gas containing fluorine. A pair of discharge electrodesandfor causing discharge-excitation of laser gas, a pulse power module (PPM)that includes a switchto cause the discharge electrodesandto perform pulse discharge, an electrical insulating unit, and a feedthroughare disposed in the laser chamber. The discharge electrodeis a cathode electrode, and the discharge electrodeis an anode electrode. The electrical insulating unitsupports the discharge electrodewith the discharge electrodeinsulated from the PPM.

The PPMincludes a charge capacitor, which is not illustrated, and is connected to the discharge electrodevia the feedthrough. The chargeris connected to the charge capacitor of the PPM. A voltage generated by the PPMis applied to the discharge electrodevia the feedthrough.

Additionally, a pressure sensor, a cross-flow fan, a shaftthat causes the cross-flow fanto rotate, a bearingthat fixes the shaft, and a motorthat provides a driving force to the shaftare disposed in the laser chamber. The pressure sensormeasures the total pressure of the laser gas. The cross-flow fanrotates within the laser chamberto circulate the laser gas. The rotation of the cross-flow fancauses the laser gas to circulate within the laser chamber.

The LNMincludes a prismthat enlarges a beam, and a grating. The gratingis disposed in Littrow arrangement such that an incident angle and a diffraction angle become the same. The OCis a partial reflective mirror coated with a multi-layered film that reflects a part of laser light generated in the laser chamberand transmits the other part. The OCforms a laser resonator together with the LNM. The laser chamberincludes two windowsandthat transmit light of the laser resonator, and is disposed on an optical path of the laser resonator.

The power monitorincludes a beam splitter, a light condensing lens, and an optical sensor, which are disposed on an optical path of laser light output from the OC.

The laser gas supply and exhaust systemincludes a laser gas supply system and a laser gas exhaust system, which are not illustrated. The laser gas supply system includes a flow rate control valve and is connected to a gas cylinder that serves as a source of the laser gas. The laser gas exhaust system includes an opening/closing valve and an exhaust pump. The laser may be, for example, Ar or Kr as rare gas, Fgas as halogen gas, Ne or He as buffer gas, or mixed gas thereof.

illustrates a configuration diagram of the laser oscillator systemrotated leftward by 90° from that illustrated in. A heat exchanger, a temperature sensor, an insulating guideand a metallic guidefor rectifying the laser gas, a pre-ionization outer electrode, a pre-ionization inner electrode, and a dielectric pipethat cause corona discharge, a ground plate, and wiringsandthat hold the ground platein the laser chamberare disposed in the laser chamber. The heat exchangerchanges the temperature of the laser gas circulating within the laser chamber. The temperature sensordetects the temperature of the laser gas.

The gas laser apparatusincludes a laser gas temperature control systemthat controls the temperature of the laser gas circulating within the laser chamber. The laser gas temperature control systemincludes a laser control processor, a temperature sensor, a heat exchanger, refrigerant pipingsandthat circulate a refrigerant inside the heat exchanger, and a chillerthat supplies the refrigerant to the heat exchangervia the refrigerant pipingsand.

The refrigerant pipingincludes a flow rate sensorand a valve. The flow rate sensordetects the flow rate of the refrigerant distributed through the refrigerant piping. Flow rate information detected by the flow rate sensoris sent to the laser control processor. The valveis a valve that can be opened and closed in response to a signal from the laser control processorand adjusts the flow rate of the refrigerant. The valvemay be an opening/closing valve or a flow rate control valve.

The chillerthat cools the refrigerant flowing inside is connected to the heat exchanger. The chillercan freely change the set temperature of the refrigerant based on an instruction from the laser control processor, thereby controlling the temperature of the laser gas inside the laser chamber.

The laser control processoris electrically connected to the temperature sensorand is capable of measuring the temperature of the laser gas inside the laser chamberbased on an output signal of the temperature sensor. The laser control processorcontrols the valvebased on the measurement result of the temperature sensor. Note that a plurality of gas laser apparatuses, which are not illustrated, may be connected to the chiller. The chillermay be connected to other apparatuses, which are not illustrated.

The laser control processorfunctions as a control device for the gas laser apparatus. The laser control processoris a processing device including a storage device that stores a control program and a central processing unit (CPU) that executes the control program. The laser control processoris specially configured or programmed to execute various kinds of processing included in the present disclosure. The storage device is a non-transitory computer-readable medium as a tangible entity and includes, for example, a memory that is a main storage device and a storage that is an auxiliary storage device. The computer-readable medium may be, for example, a semiconductor memory, a hard disk drive (HDD) device, a solid-state drive (SSD) device, or a combination of a plurality of these devices. The laser control processoris electrically connected to an exposure device control processorof an exposure device.

The laser chamberis filled with laser gas supplied from the laser gas supply system, and the laser gas continuously circulates in the direction of the white outlined arrow A ininside the laser chamberby the cross-flow fanthat rotates continuously.

The laser gas is rectified by inclined surfaces of the insulating guideand the metallic guideand is supplied to a discharge space. The discharge space includes a space between the discharge electrodesand. The flow speed of the laser gas passing through the discharge space is improved by the rectification, and thus a discharge product generated in the discharge space can be efficiently removed from the discharge space. As a result, arc discharge due to the discharge product is suppressed.

is a flowchart of a laser oscillation operation in the gas laser apparatusaccording to the comparative example. Once the laser oscillation flow illustrated inis started, the laser control processordetermines whether or not the temperature of the laser gas is being controlled in Step S. In a case where No determination is made as a determination result in Step S, that is, if the temperature control of the laser gas has not been started, the laser control processorrepeats Step S. On the other hand, in a case where Yes determination is made as a determination result in Step S, that is, after the temperature control of the laser gas is started, the laser control processormoves on to Step S. Note that the laser gas temperature control flow will be described later using.

In Step S, the laser control processorapplies a high voltage to the chargerbased on a luminous trigger signal received from the exposure device control processorand target pulse energy, thereby causing laser light to oscillate. Once the laser control processorreceives the luminous trigger signal from the exposure device control processor, the laser control processorcauses the switchin the PPMto operate to apply a high voltage between electrodes, namely between the pre-ionization outer electrodeand the pre-ionization inner electrode, which are the pre-ionization electrodes in the laser chamber, and the discharge electrodesand, which are the main discharge electrodes.

As a result, corona discharge occurs at the pre-ionization electrodes first, and discharge ultraviolet light (UV light) is generated. The laser gas is pre-ionized by the laser gas between the main discharge electrodes being irradiated with the UV light. Thereafter, main discharge occurs between the discharge electrodesand, the laser gas is excited, and laser oscillation occurs in the laser resonator including the OCand the grating.

At this time, pulsed laser light narrowed by the prismand the gratingis output from the OC. A part of the pulsed laser light output from the OCis incident on the power monitor, a part thereof is reflected by the beam splitter, and pulse energy of output laser light is detected by the optical sensorvia the light condensing lens.

The pulse energy of the output laser light detected by the power monitoris input to the laser control processor. The laser control processorintegrates a number of pulses of the laser light in a counter circuit provided in the laser control processorbased on the output of the power monitor. Note that the integration of the number of pulses may be performed based on the luminous trigger signal. The laser light having been transmitted through the beam splitteris output toward the exposure device.

In Step S, the laser control processordetermines whether or not to interrupt the laser oscillation based on a signal from the exposure device control processor. In a case where No determination is made as a determination result in Step S, that is, while the laser control processordoes not receive a signal to interrupt the exposure from the exposure device control processor, the processing returns to Step S, and the following steps are repeated. While the gas laser apparatusis operating, the laser control processorperforms feedback control on the high voltage with which the chargeris charged, based on a difference between the target pulse energy and the actually output pulse energy.

In a case where Yes determination is made as a determination result in Step S, that is, in a case where the laser control processorreceives the signal to interrupt the exposure from the exposure device control processor, the processing proceeds to Step S.

In Step S, the laser control processorstops the output of laser light to the exposure deviceby stopping the laser oscillation or causing an optical shutter to move to the laser optical path. After Step S, the flowchart inis ended.

is a flowchart of laser gas temperature control in the gas laser apparatus. Once the laser gas temperature control flow illustrated inis started, the laser control processorsets a target temperature Tof the laser gas such that pulsed laser light of target pulse energy is output in Step S. Note that the target temperature Tis a temperature of a predetermined value corresponding to the target pulse energy.

In Step S, the laser control processoracquires a current laser gas temperature Tin the laser chamberfrom an output of the temperature sensor.

In Step S, the laser control processordetermines whether or not to stop temperature control based on a signal from the exposure device control processor. In a case where No determination is made as a determination result in Step S, that is, if the laser control processordoes not receive a stop signal, the laser control processormoves on to Step Sand continues the temperature control.

In Step S, the laser control processorcompares the laser gas temperature Twith the target temperature TO and determines whether or not T=Tis satisfied. In a case where Yes determination is made as a determination result in Step S, that is, in a case where the laser gas temperature Tis determined to be equal to the target temperature T, the laser control processorreturns to Step S.

In a case where No determination is made as a determination result in Step S, that is, in a case where the laser gas temperature Tis determined not as the target temperature T, the laser control processormoves on to Step S.

In Step S, the laser control processorexecutes processing of changing a laser gas temperature control mode. In Step S, the laser gas temperature control systemexecutes an operation of changing a cooling effect on the laser gas such that the laser gas temperature Treaches the target temperature T. Details of the processing of changing the laser gas temperature control mode applied to Step Swill be described later using. After Step S, the laser control processorreturns to Step S.

In a case where Yes determination is made as a determination result in Step S, that is, in a case where the laser control processorreceives a stop signal, the flowchart inis ended to stop the temperature control.

is a flowchart illustrating a subroutine for the processing of changing the laser gas temperature control mode applied to Step Sin. Although the operation flow illustrated inassumes that the flow rate of the refrigerant supplied to the heat exchangeris controlled by opening and closing the valvein the laser gas temperature control system, the operation flow is not limited to this example, and the laser gas temperature may be controlled by a method of controlling the temperature of the refrigerant supplied from the chilleror a combination thereof. Furthermore, the opening and closing of the valvemay be simple opening/closing control or may be flow rate control that can change the flow rate in a stepwise manner or a continuous manner. The opening and closing control of the valveis an example of a method for adjusting the flow rate of the refrigerant.

Once the processing of changing the laser gas temperature control mode illustrated inis started, the laser control processordetermines whether the current laser gas temperature Tis lower or higher than the target temperature Tin Step S. For example, the laser control processordetermines whether T<Tis satisfied, and in a case where Yes determination is made as a determination result in Step S, the laser control processordetermines that the laser gas temperature Thas not reached the target temperature Tand the temperature needs to be raised, and moves on to Step S.

In Step S, the laser control processordetermines an opening and closing state of the valve. For example, the laser control processordetermines whether or not the valveis in an open state, and in a case where Yes determination is made as a determination result in Step S, that is, if the valveis in an open state, the laser control processormoves on to Step S.

In Step S, the laser control processorcloses the valveto reduce the cooling effect and stops the supply of refrigerant. The cooling effect of the heat exchangeris reduced, and the laser temperature Tis gradually raised by the supply of the refrigerant being stopped. The operation of raising the laser gas temperature Tas in Step Sis defined as a “temperature rising mode”. After Step S, the laser control processorends the flowchart inand returns to the flowchart in.

On the other hand, in a case where No determination is made as a determination result in Step S, that is, if the valveis in a closed state, this means that a state where the cooling effect is low has already been achieved, the valveis thus kept closed, and the laser control processorends the flowchart inand returns to the flowchart in.

In a case where No determination is made as a determination result in Step S, the laser control processordetermines that the laser gas temperature Tis higher than the target temperature Tand the temperature needs to be lowered, and moves on to Step S.

In Step S, the laser control processordetermines an opening and closing state of the valve. For example, the laser control processordetermines whether or not the valveis in a closed state, and in a case where Yes determination is made as a determination result in Step S, that is, if the valveis in the closed state, the laser control processormoves on to Step S.

In Step S, the laser control processoropens the valveto enhance the cooling effect and restarts the supply of the refrigerant. The cooling effect of the heat exchangeris improved, and the laser gas temperature Tgradually drops by the supply of the refrigerant being restarted. The operation of lowering the laser gas temperature Tas in Step Sis defined as a “temperature dropping mode”.

On the other hand, in a case where No determination is made as a determination result in Step S, that is, if the valveis in an open state, this means that a state where the cooling effect is high has already been achieved, the valveis thus kept open, and the laser control processorends the flowchart inand returns to the flowchart in.